Here's why:

* Metaphor: A metaphor directly compares two unlike things without using "like" or "as". For example, "He is a lion in battle."

* Simile: A simile compares two unlike things using "like" or "as". For example, "He is like a lion in battle."

In the phrase "August is a day in the sun", the comparison is being made using "is" (which functions similar to "like" or "as"). This implies a direct comparison between the month of August and a day that is filled with sunshine.

Therefore, "August is a day in the sun" is a simile.